A direct current (DC) to direct current (DC) converter converts a source of direct current (DC) from one voltage level to another. When converting an input voltage to a higher level the circuit is called a “boost” or step-up converter, and when converting into a lower level it is called a “buck” or step-down converter.
There are several types of DC to DC converters. One known DC to DC converter is a switched-mode DC to DC converter that converts DC voltage of a first level from a DC power supply by controlling a switch that may be closed to selectively allow the provision of input power (from the DC power supply) to storing the input energy temporarily in an energy storage component (such as an inductor and/or a capacitor) that releases that energy to the output of the DC to DC converter at a different level.
FIG. 1
FIG. 1 illustrates a prior art DC-DC converter 10. It includes DC power supply 11, switch 12, diode 13, inductor 14, capacitor 15, output ports 16, measurement circuit (such as voltmeter 17) and controller 18. The controller 18 controls the switch 12 in response to the value of output voltage Vo. DC to DC converter 10 is connected to a load 20.
FIG. 2
FIG. 2 illustrates some waveforms generated during the operation of DC to DC converter 10.
The switch 12 is kept closed during switch ON times (TON 21 of FIG. 2) and this causes the inductor 14 to experience a forward voltage drop VL=Vi−VO (as illustrates by graph 23) which causes the inductor 14 to be charged with energy (from power supply 11) and the current on the inductor 14 increases linearly (graph 24) assuming the inductor 14 does not reach saturation.
The switch 12 is opened during switch OFF times (TOFF 22 of FIG. 2) and this causes diode 13 to conduct and thus the voltage drop VL on inductor 14 is negative VL=VD−VO. During this time period the inductor's energy keeps a flow of current into the load 20, but this current decreases linearly as well (graph 24).
The average current the buck circuit can provide to the output load is shown in the diagram as IAVG 25. Adjusting the circuit parameters such as duty cycle and frequency of the switch 12 may change the available current to the output load. If the average output load is smaller than this average current then voltage will increase beyond required level and it will fall below it when the output load is smaller. For this reason, in order to keep a constant output voltage level a closed loop mechanism is typically implemented.
Closed Loop Mechanism.
A popular way of implementing a closed loop mechanism for keeping a stable output voltage level is by using “ON-OFF switching” mechanism. The average inductor current IAVG 25 is higher than the average load current Iload causing the output voltage VO to increase when the switching mechanism is on.
The output voltage Vo is sensed by the circuit and when voltage rises above a certain threshold the switching mechanism is stopped allowing the voltage to drop as a result of the output load. This is referred to as “Inactive Period” (Denoted 32 in FIG. 3). After the output voltage drops below a certain threshold the switching mechanism is turned on again. This is referred to as “Active Period” (denoted 31 in FIG. 3).
FIG. 3
Prior art waveforms related to the activation of a prior art switched mode DC to DC converter are illustrated in FIG. 3. For simplicity the output load is shown as constant.
The output voltage (Vo) 34 exhibits ripples that result from several factors in the system such as the hysteresis of the voltage threshold detection mechanism, the difference between IAVG and ILOAD and switching frequency.
One way of reducing output ripple is to adjust the duty cycle of the switch control in order to reduce the difference between these currents and reduce the slope of VO. When the average output current during TON is lower than the average load current then the load is in starvation and the converter cannot meet the load demand. In this case the output voltage will fall below its target.
In many electronic systems there is a dedicated ASIC component that is responsible for providing power to the different consumers. This component is referred to as Power Management Unit (PMU). As designs become more advanced the typical consumer requires increasingly larger current form the system. A typical PMU in portable application systems has several voltage converters (DC-DC modules) that convert battery voltage to different voltage levels to meet the system power demands.
As mentioned above, a given DC-DC converter can provide a certain amount of current to the load. If load is larger than the maximum average current the circuit can provide the consumer will be in starvation. In this case the closed loop mechanism will be in “Active” mode constantly.
Traditionally, if the PMU does not have a single DC-DC converter that can meet the demand of a consumer then an external component must be purchased and integrated into the system, increasing the system cost and printed circuit board (PCB) area.
Even though the PMU may have several different DC-DC converters that the sum of their available current is enough to meet the consumer demand it is not possible to simply connect them together to supply the demand. Several tests in the lab show that this can lead to output voltage instability, decreased efficiency and might cause damage to the PMU.
Some PMUs can solve this issue by synchronizing the control signals of several DC-DC converters, effectively joining them together into one big DC-DC converter with the combined size of the switches and inductor components. This, however, requires additional logic that increases cost of the PMU, and might not be available in any given PMU component.
FIG. 4
FIG. 4 illustrates typical prior art waveforms 41, 42, 43, 44, 45, 47 and 48 generated in relation to a scenario in which a load may be in a state of starvation (lack of current illustrated by regions 46). The current (44) supplied by the DC to DC converter to the load is not enough to provide the demand from the load (45). As a result the voltage Vo (42) starts to drop below the desired voltage level Vtarget (41) during the OFF period of the switch TOFF when IAVG falls below ILOAD—as illustrated by areas 44. Graph 47 illustrates a switch control signal (“switch state”) that toggles the switch of the DC to DC converter and also illustrates an ON_CTL signal (48) that shows that the switch is toggled during the entire duration.
Avoiding Oscillation on Output Voltage
Traditionally, there is a risk that two different DC to DC converters that are controlled by separate controllers and are coupled in parallel to each other (to provide the same output voltage) might cause the output voltage to oscillate.
This is the reason that traditionally using several DC-DC converters in parallel requires synchronization of the switching elements of each DC to DC converter, which complicates the design of the DC to DC converters and increases the overall cost.
FIG. 5
FIG. 5 illustrates various waveforms generated when using two DC-DC converters with close frequencies and similar current rating.
During time periods 52 the output voltage 53 is below the voltage threshold low hysteresis level, and during periods 51 the output voltage is above the high hysteresis level. The low hysteresis level is slightly below Vtarget and the high hysteresis level is slightly above Vtarget.
In each rising edge of the respective DC-DC converter a switching operation occurs if the output voltage is below the low hysteresis point (periods 51) and stops if it is above the high point (periods 52).
The bandwidth of a DC-DC converter is limited by the frequency of the switching. The decision point whether to close the switch comes usually at the beginning of each switch period.
The diagram shows output voltage instability (large Vo fluctuations) due to the fact that both controllers might decide to perform a switch in the same time while the current that is transferred to the output is a sum of both of them. Graphs 54, 55, 56, 57, 58, 59 and 59′ represent the current drained from a first DC to DC converter, the load current, the current drained from a second DC to DC converter, a first switch state control signal, a control signal that determines whether to toggle the first switch, a second switch state control signal, and control signal that determines whether to toggle the switch first switch, respectively.
There is a need to provide an efficient system and method for operating multiple DC to DC converters.